This invention relates generally to corrective spinal surgery and, in particular, to vertebral fusion devices providing a greater surface area to enhance fusion.
In conjunction with spinal surgery, interbody fusion cages are regularly placed between the endplates of the vertebrae to aid in fusion. This is justified on the basis that the end plates of the vertebrae are stronger than the cancellous bone within the body of the vertebrae, enabling the strength of the end plates to be relied upon for distraction. Such distraction restores the disc space height, thereby hopefully alleviating pressure on the spinal nerves and, hopefully, reducing pain or discomfort, if present.
Fusion is more successful if done in cancellous bone, however. Thus, when performing interbody fusion, the surgeon must balance preserving the end plates for support while removing at least a portion thereof to aid in fusion. To achieve this balance, the surgeon typically scraps or decorticates a portion of the opposing end plates to expose the underlying cancellous bone.
FIGS. 1A and 1B illustrate, from an oblique perspective, existing prior-art devices such as strut graft 100 and cages 101 used for distraction and/or fusion. It is important to note that, although FIG. 1A implies the use of strut graft 100 and cages 101 used in combination, they are used separately according to the current practice. That is, when a strut graft is used, cages are not, and when cages are used, they occupy the entire intervertebral space, thereby precluding the use of a strut graft. Nevertheless, both are shown in the figure as the alternatives currently in use. The strut graft 100 is typically an elongated body dimensioned to extend from the inferior end plate 108 of vertebrae 102 to the superior end plate 110 of vertebrae 106, also extending through an intermediate vertebrae 104 through a slot 112. Those of skill appreciate that longer members, penetrating through a plurality of slotted vertebrae are also in use. FIG. 1B illustrates the arrangement from a transverse section taken through vertebrae 104.
To install the graft 100, the channel 112 is formed into one or more intermediate vertebrae using chisels or power burrs. The walls formed through the vertebrae are accordingly irregular, reducing the likelihood of contact points 120 between the device 100 and the walls of the slot. As best seen in FIG. 1B, even if a perfectly rectangular slot were to be formed through intermediate vertebrae 104, the points of contact would be limited to points 120. The decreased contact area increases the likelihood of device migration and failure of fusion. In addition, only a small portion 109 of the inferior end plate 108 of vertebrae 102, and a correspondingly small portion (covered by the end of device) of superior end plate of vertebrae 106 is scraped away and allowed to fuse with the ends of the graft 100, the result being a structure which is less than optimal. The area for fusion adjacent to the endplates is even smaller with interbody fusion cages 103.
The subject of the present invention resides in enhanced surface area spinal fixation devices. Broadly, in contrast to existing intervertebral cages and strut grafts, which minimally penetrate the cortical endplates of the vertebra to be fused, devices according to the invention have upper and lower sections which are implanted directly within the bodies of the vertebra being fused, thereby surrounding the implanted sections with cancellous bone which is more conducive to ingrowth and permanent fusion.
In the preferred embodiment, a spinal fixation device according to the invention comprises a frame-like structure composed of biocompatible material such as carbon fiber, having a substantially hollow interior and open side walls, ends, or apertures to receive bone-graft material. The device is preferably used in conjunction with adjoining intervertebral cages, which may be rigidly joined to one another and/or to the inventive device. The cages rely on retained endplates for distraction. Wedge-shaped distraction plugs, similar to intervertebral cages, may alternatively be employed. Since fusion occurs through the device to enhance surface area, such plugs may be solid.
The spinal fixation device also preferably further includes one or more physical features to engage with surrounding bone or minimize back-out, such as teeth, ridges, grooves, or outriggers. One or more shape-memory elements may also be used, each of which is preferably compressed for insertion then expanded when the device is in place.
The inventive fixation device need not be employed between adjacent vertebra, but may be used between vertebra separated by one or more intermediate vertebra, in which case the device extends through the intermediate vertebra, preferably in intimate contact therewith. Multiple devices may also be implanted side by side between the same set of adjacent vertebra. In addition to these various alternative embodiments of the invention, a preferred method of installation is also disclosed.